Recently, along with popularization of an audio/visual (AV) device which handles a digital signal such as an audio signal and a video signal, various types of interfaces for transmitting the digital signal between the AV devices are suggested. As such interface, the Institute of Electrical and Electronics Engineers (IEEE) 1394 standard, the High-DefinitionMultimedia Interface (HDMI) (TM) standard are widely known. In such interface, technology of transmitting the audio signal in a direction opposite to a transmitting direction of a main large-capacity channel is known. For example, an interface circuit which transmits a biphase modulation signal referred to as SPDIF in a direction opposite to a transmitting direction of a serial transmission path referred to as TMDS is suggested in the HDMI standard (for example, refer to Patent Document 1).